Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing exhaust emissions. For example, natural gas has been recognized as an attractive alternative fuel. For automotive applications, natural gas may be compressed and stored as compressed natural gas (CNG) in cylinders at high pressure. As another example, alcohol and alcohol-containing fuel blends may be used as an alternative fuel for automotive applications. Various engine systems may be used with alcohol fuels and CNG fuels, utilizing various engine technologies and injection technologies that are adapted to the specific physical and chemical properties of the alternative fuels. Based on engine operating conditions, an engine control system may adjust a fuel injection profile of a multi-fuel system to take advantage of the specific properties of the available fuels. This may include operating with either one of the fuels, or using a co-fueling approach wherein both fuels are simultaneously injected. Co-fueling can provide the advantages of each fuel and may be serendipitously advantageous over either fuel during selected conditions.
One example multi-fuel system is described by Surnilla et al. in U.S. Pat. No. 7,703,435. Therein, an engine is configured to operate on CNG, gasoline, or a mixture of both. Fuel is selected for operating the engine during particular operating conditions based on the amount of fuel available in each fuel storage tank as well as based on the type and attributes of the available fuel. For example, vehicle fuel economy and range can be extended by selecting a particular fuel during high driver demand. As another example, engine emissions can be improved by reserving a particular fuel for engine starting conditions. Another approach is shown by Leone et al. in U.S. Pat. No. 8,307,790. Therein, the engine is operated with a gaseous fuel generated via reformation and a liquid fuel including an alcohol fuel blend. During conditions when a transition from liquid fuel to gaseous fuel is requested, the gaseous fuel injection is ramped in initially at a low rate. Once the gaseous fuel composition has been determined, the fuel injected is ramped in at a higher rate.
However the inventors herein have recognized potential issues with such approaches. As an example, transient fuel issues may occur. These issues may be exacerbated when transitioning between co-fueling modes. The transient fuel issues may be complex to track due to the use of multiple fuels and multiple injection technologies. For example, transient fuel issues experienced when transitioning from direct injection of a first liquid fuel to direct injection of a first and second gaseous fuel may be substantially different from those experienced when transitioning from direct injection of a first liquid fuel to direct injection of a first fuel and port injection of a second gaseous fuel. Furthermore, due to transient fuel effects, when transitioning from co-fueling with a first fuel split to co-fueling with a different fuel split, there is a risk of not matching the engine control correctly with the actual fuel present in the cylinder. The variability in the fuel chemistry can make the fuel transition even more difficult. For example, the quality of fuel (e.g., CNG fuel) can vary radically from tank fill to tank fill. As a result, a rapid shift in fuel split can compromise the control system's ability to compensate for transient fuel issues, resulting in torque losses, abnormal combustion (e.g., knock or misfire), and degraded fuel economy. Due to the transient fuel issues, a controller may not enable co-fueling even during conditions where co-fueling provides serendipitous advantages over usage of either fuel individually. As a result, co-fueling opportunities may be missed.
In one example, some of the above issues may be addressed by a method comprising: transitioning from operating an engine with a first split ratio of a first fuel and a second fuel to operating with a second, different split ratio, a rate of change in split ratio during the transitioning limited per engine cycle. In this way, by limiting the rate of fuel split change allowed at each engine cycle, transient fuel issues incurred during co-fueling can be reduced.
As an example, an engine may be configured with a multi-fuel system so as to operate on a first, liquid fuel, such as gasoline, and a second, gaseous fuel, such as CNG. During selected operating conditions, the engine may be operated with at least some CNG and at least some gasoline injected into the engine cylinders to provide benefits that minimize the consumption of liquid fuel while meeting the torque demand. For example, during conditions of a torque band (where engine speed is in the range of 3000-4500 rpm and engine load is high), the engine may be co-fueled with a fuel split of 70% gasoline and 30% CNG and an overall combustion air-fuel ratio that is 30% rich so as to meet the torque demand while also allowing spark to be maintained at MBT. As another example, during conditions of a power band (where engine speed is in the range of 4500-6000 rpm and engine load is high), the engine may be co-fueled with a fuel split of 15% gasoline and 85% CNG (and an overall combustion air-fuel ratio that is 10% rich) so that a small amount of gasoline at a smaller amount of richness, in addition to the CNG, can restore full power, while maintaining spark at MBT. When transitioning from co-fueling at the first fuel split ratio used in the torque band conditions to co-fueling at the second fuel split ratio used in the power band conditions, a controller may limit the rate at which the fuel split ratio is changed. Specifically, instead of transitioning from the first fuel split ratio of 70% gasoline and 30% CNG to the second fuel split ratio of 15% gasoline and 85% CNG substantially immediately (e.g., over a single engine cycle), the controller may limit the change to a threshold percentage per engine cycle to reduce potential disturbances from the wholesale change. The rate at which the split ratio is changed at each engine cycle may be limited based on engine operating conditions such as engine temperature, MAP, fuel volatility, fuel availability, change in injection type, etc. As an example, the controller may gradually increment the CNG ratio by 5% over each engine cycle while correspondingly decreasing the gasoline ratio by 5% until the desired fuel split ratio is achieved.
In this way, transient fuel effects incurred in a multi-fuel system when transitioning between co-fueling modes can be reduced. In addition, it may be easier to compensate for the reduced transient fuel effects using engine controls (e.g., throttle control, spark control, etc.). By reducing the transient effects, the use and advantages of a co-fueling approach during engine operation can be extended over a wider range of operating conditions. As such, this improves fuel economy. In addition, degraded cylinder combustion and abnormal cylinder combustion events arising due to the transient fuel effects are reduced, improving overall engine performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.